Often considered one of Lord Voldemort's most loyal Death Eaters, Barty Crouch Jr. is the definition of "taking the wrong path". The son of one of the wizarding world's most prominent figures, Barty Crouch Sr., Junior had every opportunity available to him, but eventually he grew to despise his father for the way he treated his mother. At the young age of 19, Junior joined Lord Voldemort and was quickly apprehended and convicted of torturing Frank and Alice Longbottom into insanity along with Bellatrix Lestrange. Though many believed Sirius Black was the first to escape the walls of Azkaban, Crouch was actually first, and his father smuggled him out as a final favor to his dying wife. Living prisoner in his own home, Junior waited patiently for his master to come for him. And in 1994, that's what happened. While Voldemort held his father under control, Crouch impersonated Hogwarts professor Alastor Moody and gained the trust of Harry Potter, eventually fixing the Triwizard Tournament so that Harry would win, transporting him into Lord Voldemort's rebirth. Voldemort considers Crouch his most loyal Death Eater, even after Crouch suffers a horrible fate just hours after Voldemort's rebirth.
In the films, Crouch is one of the characters that has been almost butchered, as his demise is never explained in the fourth installment. He only appears in Harry Potter and the Goblet of Fire, both the book and film, played by David Tennant.
**Plot points involving Crouch that were left out of/changed in the film.
1) Barty Crouch Jr. kills his father. Though we see Crouch Sr. die, we never find out who committed the murder. In the book, his son reveals himself as the murderer.
2) In the book, Crouch Jr. begs for forgiveness at his trial, screaming regret and sobbing hysterically. In the film, he is portrayed as completely insane.
3) In the film, we are led to believe he is returned to Azkaban...not true. Crouch suffers the Dementor's Kiss shortly after questioning by Dumbledore and Snape, and is worse than dead, thus negating any chance he would ever return in the series.
4) In the film, we see Crouch Jr. as himself in the very first scene of the movie, while he does not appear as himself in the present time until the end of the book.